1. Field of the Invention
This invention relates to a radiation image recording and reproducing method, and more particularly to a radiation image recording and reproducing method using a stimulable phosphor.
2. Description of the Prior Art
As disclosed in U.S. Pat. Nos. 3,859,527, 4,236,264, 4,276,473, and 4,315,318, it has been proposed to use a stimulable phosphor in a radiation image recording and reproducing method. The stimulable phosphor is able to store radiation energy upon exposure thereof to a radiation and then emits light in proportion to the stored energy upon stimulation by an electromagnetic wave selected from visible light and infrared rays. The radiation referred to here includes electromagnetic waves or corpuscular rays, e.g. X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, high-energy neutron rays, electron beams, vacuum ultraviolet rays, ultraviolet rays and the like. In the radiation image recording and reproducing method, the stimulable phosphor is first exposed to a radiation passing through an object to have a radiation image stored therein, and is then scanned with an electromagnetic wave selected from visible light and infrared rays, which causes the stimulable phosphor to emit light in the pattern of the stored image. The light emitted by the stimulable phosphor upon stimulation thereof is photoelectrically detected and converted to an electric signal by use of a photoelectric converter such as a photomultiplier. The electric signal is processed as desired to reproduce a visible image of a quality suitable for viewing, particularly diagnostic purposes. This radiation image recording and reproducing method using the stimulable phosphor is advantageous over the conventional radiography using a photographic film provided with an emulsion layer containing a silver salt photographic material and an radiographic intensifying screen in that the image sharpness and the resolution of the obtained radiation image are excellent.
As a fluorohalide phosphor, there is known a bivalent europium activated barium fluorohalide phosphor represented by the following general formula: EQU BaFX:aEu.sup.2+
in which X is at least one of Cl, Br and I, and "a" is a number satisfying 0&lt;a.ltoreq.0 2. When stimulated with X-rays, ultraviolet rays, electron beams or the like this phosphor instantaneously emits near ultraviolet rays of high luminance. Therefore, this phosphor is used as the phosphor for radiographic intensifying screens. Further, when exposed to a radiation, this phosphor stores the radiation energy therein Then, when the phosphor carrying the radiation energy is exposed to an electromagnetic wave having a wavelength within the range of 450 nm to 1,100 nm, the phosphor emits near ultraviolet rays of high luminance in proportion to the radiation energy stored therein. Accordingly, this phosphor can be used for the radiation image recording and reproducing method, as disclosed in U.S. Pat. No 4,239,968.
In case the aforesaid radiation image recording and reproducing method is used for medical diagnosis, it is desired that the sensitivity of the system be as high as possible in order to reduce the radiation dose applied to a patient. For this purpose, the stimulable phosphor used in the method is desired to emit light having as high luminance as possible upon stimulation thereof. Thus, a need exists for an improvement in the sensitivity of the radiation image recording and reproducing method using the BaFX:Eu.sup.2+ phosphor. Namely, a need exists for an improvement in the luminance of light emitted by the BaFX:Eu.sup.2.sup.+ phosphor upon stimulation thereof